An impact modifier used to improve the impact resistance of a vinyl chloride resin is exemplified by methyl methacrylate butadiene styrene (MBS) resin, ethylene chloride (CPE) resin and acrylic resin.
To improve the impact resistance of a thermoplastic resin, a rubber polymer having as low a glass transition temperature (Tg) as possible is used. Among the rubber polymers, polybutadiene rubber added to HIPS, ABS and MBS resin has a glass transition temperature of about −80° C., which is much lower than the glass transition temperature (−50° C.) of polybutylacrylate which is a rubber component used as an acrylic modifier, suggesting that the polybutadiene rubber has excellent impact resistance. However, the polybutadiene rubber contains unsaturated double bonds, indicating that it shows weak thermal stability and low weatherability
In the meantime, acrylic resin which does not include unsaturated double bonds has excellent weatherability, so that it has been widely used as an impact modifier for outdoor plastic products which happen to be exposed to sunshine for a long time. For example, for products requiring excellent impact resistance and weatherability together, like a window frame etc, a polymer having a core-shell structure is applied, which is prepared by graft-copolymerization of methacryl polymer, which has excellent compatibility with vinyl chloride resin, with a rubber core composed of alkyl acrylate. However, the glass transition temperature thereof is higher than that of butadiene rubber, so the enhancement of impact resistance is insufficient.
Polyorganosiloxane, which is also called polydimethylsiloxane, has a glass transition temperature (Tg) of approximately −120° C., suggesting that this polymer is very effective for the enhancement of impact resistance. Thus, acryl-silicon rubber complex graft copolymer particles containing silicon can be effectively used to improve weatherability and impact resistance.
Major factors affecting the properties of the acryl-silicon rubber complex impact modifier having a core-shell structure are the rubber content in the impact modifier, the size of the rubber particles, the distance between rubber particles, the swelling index for a solvent, and the degree of coupling between the impact modifier particles dispersed by processing and a matrix. In particular, the bond between an impact modifier and a matrix depends on the shell-graft efficiency for the impact modifying rubber core.
The acryl-silicon rubber complex impact modifier for improving impact resistance of a vinyl chloride resin is prepared by the conventional emulsion polymerization, which is described in the following references.
First, U.S. Pat. No. 5,132,359 describes how polydimethylsiloxane rubber seed is polymerized, followed by growing the rubber particles with a butyl acrylate monomer added as a rubber core, and then a shell-forming component is added to cover the surface of the core to complete the core-shell structure.
Second, U.S. Pat. No. 6,153,694 describes polydimethylsiloxane rubber latex, which is 100 nm or less, and polybutylacrylate latex being polymerized respectively, rubber particles are grown by agglomeration, and then a capsulated shell is formed, leading to microagglomeration to form the core-shell structure.
The problem with the above two methods is a long polymerization time due to the separate silicon polymer polymerizations. Particularly, to prepare silicon rubber, polymerization is performed by using cyclic organosiloxane as a precursor and organosilane as a cross-linking agent in the presence of an acid catalyst, and then separate neutralization and aging are required.
During the shell-polymerization, methylmethacrylate monomer having excellent compatibility with vinyl chloride resin is graft-polymerized onto the core surface or graft-polymerized with a small amount of another monomer having at least two functional groups. In particular, methyl methacrylate has excellent compatibility with a matrix and a high glass transition temperature, so it can play a role in improving the agglomeration of latex. To increase the dispersibility of an impact modifier in a matrix, a small amount of acrylonitrile monomer is effectively added during the shell polymerization.